Fixed ratio transmissions, regardless of whether they are manual or automatic, include a discrete number of transmission ratios or gears. In comparison with conventional transmissions, in a continuously variable transmission or CVT, the transmission ratio between the input and output shafts may be progressively varied in a specific interval of possible ratios. The possibility of incorporating an unlimited number of transmission ratios provides an added parameter in order to optimize one or several variables of the vehicle. This way, with a specific variation in the transmission ratio, we can achieve conditions of high power, low consumption or a compromised ratio between both variables.
One possible classification establishes two large groups of continuously variable transmissions, the kinematic and dynamic types. In a kinematic CVT, the progressive change in transmission ratio is carried out by actuating on a specific element, resulting in the transmission ratio being fixed at a specific value and requires acting upon the element once again in order to change it. On the other hand, in a dynamic type CVT, as well as being able to act upon a kinematic regulating element, the transmission ratio also depends on the external conditions to which the transmission is subjected to. This means that the transmission ratio will be determined by the kinematic characteristics as well as by variables such as the speed of the input shaft or the resistant torque exerted on the output shaft.
The dynamic type continuously variable transmissions originate from the innovative work carried out by Hunt, which were published in GB patents 21,414 of 1912 and GB Patent 19,904 of 1913, where an inertial type transmission system is described with the dynamic transmission principle but without a direct application like CVT.
The first documented continuously variable type dynamic transmission originates from the work of Constantinesco, described in his GB Patent 185,022 of 1922 and in his subsequent patents, which describe methods for improving the power transmission of the primary shafts of vehicles that operate using internal combustion engines. In these transmissions, the torque is regulated using a pendulum or other inertial elements.
Chalmers, in U.S. Pat. No. 1,860,383 of 1932 introduces his oscillating torque transmission system with movement of the output shaft in a single direction of rotation. In this case, the regulating element consists of a series of planet gears with eccentric masses that generate an oscillating torque at the output due to the inertial forces these masses are subjected to. Similar type transmissions were designed by Tam, 1992 (U.S. Pat. No. 5,134,894) and Fernandez, 1998 (U.S. Pat. No. 5,833,567), which were also based on planet gears with eccentric masses. Also with oscillating masses, but in this case without these carrying out complete rotations about themselves, we have the torque converter patent of 1982 owned by Shea (U.S. Pat. No. 4,336,870); this transmission includes two symmetric masses shaped like a cam that oscillate, thus regulating the torque of the output shaft. Also based on inertial regulating using eccentric masses we have William's torque converter of 1971 (U.S. Pat. No. 3,581,584).
Two dynamic type transmission patents exist later to the one mentioned above that have the same principle of operation thought they use different technical solutations. The first one is U.S. Pat. No. 5,860,321 of 1999 provided by Williams, where he proposes new solutions for rectifying movement using a differential rectifier with two free wheels as well as specific configurations and new technical solutions focussed on increasing the compactness as well as the efficiency of the power transmission. The second transmission of this type provided by Lester in 2000 (U.S. Pat. No. 6,044,718) proposes solutions among which a power transmission regulating system stands out. The complete system is a CVT with inertial regulating and with the possibility of being coupled.
A large number of dynamic transmissions need a subsystem that transform a continuum rotational MOTION INTO A RATCHETING MOVEMENT. There are several mechanisms able to perform this task, from the simplest ones as a four-bar mechanism or a system of grooved plates, to more sophisticated devices as those patented by Sullivan (US 2001/0047925),Chio (U.S. Pat. No. 7,926,462B2, US 2008/0210026A1), Ozdamar (U.S. Pat. No.6,857,412B2) and Marchou (US 2088332A).